To reduce what is termed the "gap loss" between a magnetic head and a magnetic tape in a magnetic recording medium for use in high density recording since increase of "gap loss" causes reproduced output and C/N to decrease, it is necessary to improve the surface characteristics of the magnetic layer of the medium. For this purpose, as well as improvements in techniques for the formation of the magnetic layer, such as dispersion of magnetic particles, coating and the formation of a surface, an improvement in the surface characteristics (i.e., the surface smoothness) of a support is also needed. As the recording density is increased, the recording wavelength is decreased. Thus, in order to avoid thickness loss which results in the deterioration of reproduced output and C/N, an attempt to decrease the thickness of the magnetic layer has been made. When this is done, however, the surface characteristics of the support exert increased influences on the properties of the magnetic layer. The above surface characteristics of the support is described in U.S. Pat. Nos. 3,876,608, 4,461,797 and 4,489,117.
Improving the surface characteristics (i.e., the surface smoothness) of the support is limited by two factors. First, if the surface characteristics (i.e., the surface smoothness) of the medium are good, at the production step where the medium is shaped into a tape and wound, the friction resistance of the tape to a conveying roller is increased and thus, in many cases, the tape moves in a zigzag direction and wrinkles. Second, the friction resistance between the tapes is increased, thereby producing an unevenness in the shape of a winding roll.
In order to overcome the above contradictory problems, various attempts have been made. Japanese Patent Application (OPI) No. 109605/78 (The term "OPI" as used herein means a "published unexamined Japanese patent application"), for example, describes a method of forming a magnetic layer in which finely divided thermoplastic resin particles are provided on a support in such a manner that the particles project from the surface of the support and are then removed by dissolving with a solvent to improve running properties of the magnetic layer and, thereafter, the magnetic layer is formed on the surface of the support.
Japanese Patent Publication No. 14555/71 describes a method in which a solution of a polymer such as polyamide and polyester is coated on a support and dried to form fine wrinkles, and a magnetic layer is formed on the finely wrinkled polymer coating. Japanese Patent Publication No. 6117/72 describes the same method as described in Japanese Patent Publication No. 14555/71, but using copolyesters, for example, as the polymers to be coated on the support. Also Japanese Patent Publication No. 38001/75 describes the same method as described in Japanese Patent Publication No. 14555/71, but using thermoplastic polyesters as the polymers to be coated on the support.
All the above methods, however, fail to produce a magnetic recording medium having satisfactory characteristics for high density recording.